1. Field of the Invention
The present invention relates generally to molds for forming glass sheets and, more specifically, to a mold assembly for forming a glass sheet.
2. Description of the Related Art
It is known to heat glass sheets using a "hearth" or "lehr." Generally, the lehr is a furnace and may be of a continuous roller-type, fixtured roller-type or gas-type. For example, a continuous roller-type lehr has a plurality of rollers disposed beneath a plurality of radiant heaters. Typically, a glass sheet is placed inside the lehr where it is heated by conventional radiation, convection and conduction heat. The glass sheet is moved along the rollers at a predetermined rate which depends on the thermal conductivity of the glass sheet to reach a temperature in its forming range. When the glass sheet is at a temperature in its forming range, air pressure is used to move the glass sheet off the rollers and upward against a surface of mold. The mold has its surface covered with a liner of either ceramic paper or woven cloth to prevent the mold from marking the glass sheet. The mold and liner are perforated and a vacuum source is applied to the mold to hold the glass sheet against the liner of the mold. A fixture for forming the glass is shuttled in and placed on the rollers beneath the glass sheet. Once in place, in a quick sag process, the vacuum is discontinued and the glass sheet drops onto the fixture. The glass sheet is formed by its own weight to a predetermined shape of the fixture. In a press foam process, the glass sheet is pressed into the fixture by the mold to form the glass sheet to a predetermined shape. Once formed, the vacuum is discontinued and the mold removed from the fixture. The glass sheet is then cooled.
Although the above liner has worked well to prevent the mold from marking the glass sheet, it suffers from the disadvantage that the ceramic paper is fragile, wears and requires frequent replacement. The above liner also suffers from the disadvantage that the woven cloth tends to tear when contacted by an edge of the glass sheet and curl up, requiring replacement. As a result, there is a need in the art to provide a durable liner for covering a mold to prevent marking of a glass sheet when held against the mold.